Enzootic pneumonia in swine, also called mycoplasmal pneumonia, is caused by M.hyo. The disease is a chronic, non-fatal disease affecting pigs of all ages. Infected pigs show only mild symptoms of coughs and fever, but the disease has significant economic impact due to reduced feed efficiency and reduced weight gain. Enzootic pneumonia is transmitted from pig to pig through the nasal passages by airborne organisms expelled from the lungs of infected pigs. The primary infection by M.hyo may be followed by secondary infection by other mycoplasma species (Mycoplasma hyorhinis and Mycoplasma flocculare) as well as other bacterial pathogens.
M.hyo is a small, prokaryotic microbe capable of a free living existence, although it is often found in association with eukaryotic cells because it has absolute requirements for exogenous sterols and fatty acids. These requirements generally necessitate growth in serum-containing media. M.hyo is bounded by a cell membrane, but not a cell wall.
The physical association of mycoplasmas with the host cell surface is the basis for the development and persistence of enzootic pneumonia. M.hyo infects the respiratory tract of swine, colonizing the trachea, bronchi, and bronchioles. The mycoplasma produces a ciliostatic factor which causes the cilia lining the respiratory passages to stop beating. Eventually, the cilia degenerate, leaving the pig prone to infection by secondary pathogens. Characteristic lesions of purple to gray areas of consolidation are observed in infected animals. Surveys of slaughtered animals revealed lesions in 30 to 80% of swine. Results from 37 herds in 13 states indicated that 99% of the herds had hogs with pneumonia lesions typical of enzootic pneumonia. Therefore, the need for effective preventative and treatment measures are great.
Antibiotics such as tiamulin, trimethoprim, tetracyclines and lincomycin have some benefit, but are expensive and require prolonged use. Additionally, antibiotics have not been shown to effectively eliminate spread or reinfection of M.hyo. Prevention by maintaining pathogen-free herds is sometimes possible but reintroduction of M.hyo often occurs. Due to the serious economic consequences of swine pneumonia, vaccines against M.hyo have been sought. Vaccines containing preparations of mycoplasmal organisms grown in serum-containing medium have been marketed, but raise concerns regarding adverse reactions induced by serum components (such as immunocomplexes or non-immunogenic specific proteins) present in the immunizing material. Other attempts to provide M.hyo vaccines have been successful, but the disease remains widespread.
M.hyo and porcine circovirus type 2 (PCV2) are the two most prevalent pathogens that are encountered in the pig industry. Swine infected with PCV2 exhibit a syndrome commonly referred to as Post-weaning Multisystemic Wasting Syndrome (PMWS). PMWS is clinically characterized by wasting, paleness of the skin, unthriftiness, respiratory distress, diarrhea, icterus, and jaundice. In addition to PMWS, PCV2 has been associated with several other infections including pseudorabies, porcine reproductive and respiratory syndrome (PRRS), Glasser's disease, streptococcal meningitis, salmonellosis, postweaning colibacillosis, dietetic hepatosis, and suppurative bronchopneumonia. M.hyo is associated with enzootic pneumonia and has also been implicated as one of the major co-factors in the development of Porcine Circovirus Associated Disease (PCVAD).
Porcine reproductive and respiratory syndrome (PRRS) is caused by an arterivirus, which has a particular affinity for the macrophages particularly those found in the lung (alveolar macrophages). These macrophages ingest and remove invading bacteria and viruses, but not in the case of the PRRS virus. In the case of the PRRS virus, it multiplies inside the macrophages producing more virus and kills the macrophages. Once PRRSV has entered a herd, it tends to remain present and active indefinitely. Up to 40% of the macrophages are destroyed, which allows bacteria and other viruses to proliferate and do damage. A common example of this is the noticeable increase in severity of enzootic pneumonia in grower/finisher units when they become infected with PRRS virus. More than half of weaning-age PRRS virus-negative pigs become infected before going to market.
What is needed is an improved vaccine against mycoplasma infection in swine. Preferably, the M.hyo vaccine will be compatible with other porcine antigens, such as PCV2 and PRRS virus, whether they are given concurrently as separate single vaccines or combined in a ready-to-use vaccine. It would be highly desirable to provide a ready-to-use, single-dose M.hyo/PCV2 combination vaccine.